Reddit started as a simple website, but they published an API which allowed people to make their own reddit apps.
This benefitted reddit because it increased their userbase and minimized bandwidth usage because users wouldn't have to reload every time they wanted to expand a reply.
With time, third party apps became more user friendly and added more features. Everyone was happy.
A few weeks ago, the CEO, whose reddit handle is "Spez", decided reddit would hold an IPO to go public this year; in preparation, they need to make reddit super-profitable. Third party apps would have to pay per API call. The problem is that the billing was arbitrary and the amount of payment would be in the millions of dollars per year. Because third party apps can't pay, they'll have to close shop.
Third party apps not only helped ordinary users, subreddit moderators depend on them to efficiently curb abuse and spam (because the default app SUCKS. It's slow, overloads you with ads and has zero accessibility features). Some of these mods help maintain marginalized communities such as r/traa (transgender meme community), and the mods stated that without these tools they'll have to close the subreddits.
But Spez still doesn't give a shit, and worse, his replies are blatantly cynical. He admits he doesn't give a shit, is only focused on the money, and if you suffer for the changes, then sucks to be you. (he didn't say that literally, but more or less that's the gist of it). He's also lied, downplayed, accused opposers of wrongdoing and deflected uncomfortable questions.
Spez also accused the author of Apollo (a third party app) of blackmailing him when he just exposed his lies on the internet.
(Also, this is not the first time Spez does something despicable and failed to give a reasonable reply to the community. He once was caught red-handed using his admin powers to EDIT posts made by a disgruntled user.)
Mods of around a thousand different subreddits recently organized a blackout in which they'll close posting to their communities (or make them private) in protest for the policy changes.
Creators of alternative decentralized platforms such as Lemmy and Kbin started advertising and organizing. Some of them were banned for no legitimate reason. People have started to get really angry. Some are threatening to close their reddit accounts. One even threatened suicide because they depend on reddit to cope for their MH issues.
Meanwhile, lemmy dot ml (the main Lemmy website, akin to mastodon dot social) experienced a spike in new user sign ups and the entire lemmy network is overloaded to the point of failure.
TL;DR: The reddit CEO is a greedy asshole and is incapable of listening to the community. The community protested and the CEO has started to ban people promoting the competition. Now everyone is angry and have started to migrate. Others feel hopeless and trapped in a platform that has disenfranchised them.



